1. Technical Field
This invention relates to signaling apparatus and more specifically to telephone signaling apparatus which generate pulses of electric current for transmission over a telephone line.
2. Description of the Prior Art
It is well known in the art to use dial pulsing telephone sets in combination with pulse responsive switching equipment at a central office. In such sets circuitry operate to momentarily break or interrupt the connection of the tip and ring lines forming the subscriber loop between the central office and the telephone set. These interruptions of a direct current provided by the central office are representations of called numbers being transmitted from the telephone set to the central office.
In sets having pushbutton-type number entry, the dial pulsing is generally done asynchronously with the number entry since the number entry can occur at a much higher rate of speed than the dial pulses can be accepted at the central office. The telephone set thus stores the dialing information and activates a pulsing circuit in accordance with this information in a proper sequence. Each stored number comprises a pulse train and is transmitted at the rate required by the pulse responsive switching equipment at the central office where pauses between the pulse series define different numbers.
Operating power is not available from the central office during the break intervals. And those telephone sets requiring this power should have internal circuitry with low power requirements and minimal circuitry in the subscriber loop that reduces the voltage available to the telephone set. This is desirable since a local electrical store, such as a capacitor, is generally utilized to retain over the dial pulsing period the voltage required to operate the internal circuitry during the break intervals.
Various dial pulse arrangements have been proposed and used heretofore employing a relay to momentarily break the connection of the tip and ring lines between the central office and the telephone set. An example of such an arrangement is seen in U.S. Pat. No. 4,046,971, issued to H. Losehand on Sept. 6, 1977. Relays as a rule, however, consume considerably more current than solid-state switches and are less preferable where low power requirements exist.
Recent advances in solid-state technology allow for producing electronic telephones that avoid the use of relays in providing the dial pulsing functions. An example of such a circuit arrangement appropriate for telephone outpulsing is disclosed in U.S. Pat. No. 3,973,084, issued to P. J. Hovland on Aug. 3, 1976. The circuit has solid-state switching, but also includes a polarity guard to maintain a single polarity voltage from a central office. While such an arrangement has been satisfactory where there is ample loop current available from the central office, nevertheless, at low loop current levels the voltage drop across the polarity guard circuit becomes a significant factor in limiting the voltage available to the telephone set.
In an effort to overcome this disadvantage, it has been proposed heretofore to substitute Schottky barrier diodes for the silicon diodes currently being used in polarity guard circuits. Although these diodes have a lower series resistance than silicon diodes, they also reduce the voltage reaching the telephone set from the central office and similarly become significant at low loop current levels. Another proposal for minimizing the effect of the polarity guard circuit on the telephone set, and some of the circuitry therein, has been to move the polarity guard circuit to a point in the loop path so that it is in front of the signaling electronics in the telephone set but after the speech network. See Victor Godbole "Semiconductors Provide New Features," Telephony (Oct. 29, 1979), pages 60-101. However, since an active speech network is replacing the conventional passive speech network in new electronic telephones, the polarity guard circuit must also be located in front of the active speech network as well to protect it against voltage polarity reversals from the central office.